As indicated in U.S. Pat. No. 4,078,286, in a typical process for electrophotographic duplication, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member, and the latent image is subsequently rendered visible by the application of electroscopic particles, which are commonly referred to as toner. The visible toner image is then in a loose powdered form and it can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or another support such as a sheet of plain paper. The present invention relates to the fusing of the toner image upon a support.
In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
The use of thermal energy for fixing toner images onto a support member is well known. Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a flat or curved plate member in pressure contact with a roll; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the materials being copied there. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquify and a splitting of the molten toner takes place during the fusing operation. "Cold offset" may be caused, even at the temperatures below the molten point of the toner, by such factors as imperfections in the surface of the fusing members; by the toner particles being insufficiently adhering to the support; by electrostatic forces which may be present; etc.
Another problem frequently encounteres in fusing with a heated member is that the substrate, e.g. a sheet of paper, on which the toner image is fused may curl and/or adhere to the heated fuser. Such adhering paper will tend to wrap itself around the fuser and thus prevent the fuser from performing its intended operations in subsequent copying cycles. Such adhering paper must be generally removed by hand, resulting in much manual labor and machine downtime.